Focused ultrasonic transducers are used for non-destructive ultrasonic inspection of component parts placed in an acoustic coupling medium.
Ultrasonic transducers of this type are known in the art.
For example, FR-A-2 650 144 shows a focused ultrasonic transducer which comprises a focusing acoustic lens and a planar piezoelectric pellet, the lens being planar-concave and placed against the pellet by its planar surface, its output surface being such that the incident ultrasonic beam emerging from the pellet is perpendicular to the plane of the latter and is refracted on the output surface in such manner that the refracted beam produced in the coupling medium, in turn refracted on the surface of the part to be inspected, converges exactly at the inspection point of the part.
In FR-A-2 650 144, the output surface of the lens is produced on the basis of calculations of two radii of curvature, but, when the surface is machined with the aid of such calculations, only a very rough approximation of the ideal Fermat surface (equiphase) is obtained, since the error increases with the distance from the reference point of the calculation of the circle and is maximal at the points which are the most remote from the reference points.
This results in more or less serious problems as to the quality of the inspection of the parts.